1. Field of the Invention
The present invention relates to surgical methods and devices to treat back and leg pain, and in particular to two-part prosthetic nucleus replacements for surgical insertion within the annulus fibrosis. Such prosthesis replaces a portion of a damaged spinal intervertebral disc to restore function.
2. Description of Related Art
The principal function of the disco-vertebral joint in the human spine is to transmit compressive loads and still allow flexibility. Adjacent vertebrae are joined by a triple-joint complex. The anterior complex, or column, is formed by the vertebral bodies which are shaped like flattened cylinders with discoid and ovoid shaped intervertebral discs sandwiched between each vertebral body. Facet joints in the rear of each vertebra have a smooth cartilage surface, lubricating joint fluid, and a covering capsule. The facet joints restrict the disc to small degrees of flexion and extension, limit rotation, and protect against translational shear stress. The disc itself comprises two principle parts, the nucleus pulposus at the core, and the annulus fibrosis, which is a multilayer bias-ply wrapping that surrounds the nucleus. The nucleus starts early in life as eighty percent water, and slowly desiccates with age or as a result of injury.
A damaged disc can cause nerve dysfunction and debilitating pain in the back, legs and arms. Typical treatments that provide relief and allow patients to function again include back braces, medical treatment, physical therapy and surgery to remove the disc. A conventional surgical solution removes the bad disc and promotes new bone growth in the space to fuse the adjacent vertebrae together.
Several different prosthetic intervertebral disc devices are described by Casey K. Lee, et al., in xe2x80x9cProsthetic Intervertebral Disc,xe2x80x9d Chapter 96, The Adult Spine: Principles and Practice, Raven Press, Ltd., New York, (copyright) 1991. The conclusion of Lee, et al., is that xe2x80x9cAn appropriately designed and fabricated prosthetic intervertebral disc may provide an improved alternative to currently available surgical approaches to low back disorders.xe2x80x9d Lee, et al., describe their work at the orthopedic research laboratories at the New Jersey Medical School xe2x80x9cto produce a prosthetic intervertebral disc design that has biomechanical characteristics similar to the natural disc.xe2x80x9d One result has been the manufacture of a unit with a nucleus, annulus, and end plates that are molded under heat and fused into a single prosthetic disc. However, success of such a device depends on solid bone attachment. Most prior concepts have been excessively complex and never used.
A prosthetic nucleus replacement can be surgically implanted within the annulus fibrosis. The natural attachments of the annulus would therefore be able to produce the requisite tensile strength of the repaired site. The prosthetic nucleus replacement would be subject primarily to compressive forces.
Such a prosthetic nucleus replacement is described by the present inventor, Kevin Lawson, in U.S. Pat. No. 6,146,422, issued Nov. 14, 2000. But this device is composed of one homogeneous material and thus its top and bottom sides exhibit identical material characteristics. The described construction can also be unnecessarily expensive to produce and difficult to fabricate.
An object of the present invention is to provide a prosthetic nucleus replacement that is useful and functional.
Another object of the present invention is to provide a prosthetic nucleus replacement that allows for bone in-growth on the inferior surface.
A further object of the present invention is to provide a prosthetic nucleus replacement that allows for a press-fit interface with the inferior vertebral bone.
Briefly, a prosthetic nucleus replacement embodiment of the present invention comprises a rounded ceramic domed body with a receptacle in its bottom side. A keeled base of titanium or other biocompatible metal is press fit into the receptacle to complete a two-piece assembly. A keel part of the base especially is finished in porous coated metal and shaped to promote natural in-growth of bone from the inferior vertebrae.
An advantage of the present invention is that a prosthetic nucleus replacement is provided that supports the normal compressive loads experienced by natural vertebrae.
Another advantage of the present invention is that a prosthetic nucleus replacement is provided that fixes well to the inferior vertebrae it sits upon.
A further advantage of the present invention is that a prosthetic nucleus replacement is provided that slides easily under the superior vertebrae it supports.
A still further advantage of the present invention is that a prosthetic nucleus replacement is provided that has optimal top and bottom characteristics by virtue of its joining of two dissimilar top and bottom materials.
A further advantage of the present invention is that a prosthetic nucleus replacement is provided that has a two-piece construction which allows for a wide variety of implant alignments.
The above and still further objects, features, and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, especially when taken in conjunction with the accompanying drawings.